Photolithography is a process that is used to create small structures, such as integrated circuits and micromachines. The photolithography process entails exposing a radiation-sensitive substance, called photoresist, to a pattern of light or other radiation. The pattern is typically created by passing the radiation through a mask, which is composed of a substrate with a pattern on its surface. The pattern blocks some of the radiation or changes its phase to create exposed and unexposed areas on the radiation-sensitive material. In a binary intensity mask, the pattern is made of a light absorbing material on an otherwise transparent substrate. In a phase shift mask (“PSM”), the pattern consists of material that shifts the phase of the light passing though it to create an interference on the photoresist that produces a sharp image. The image produced on the photoresist is referred to as the “aerial image” of the mask. The size of the structure that can be produced is limited by the wavelength of radiation used; shorter wavelengths can produce smaller structures.
As photolithography processes are called upon to produce ever-smaller structures, lithography systems are being developed that use smaller wavelengths of radiation, including infra-red and even x-ray radiation. (The terms “light” and “photolithography” are used in a general sense to also include radiation other than visible light.) Systems are now being developed that can produce structures having dimensions of 70 nm and smaller. Such structures can be fabricated by photolithography using light having a wavelength of 193 nm or 157 nm. Some photolithography masks used with such short wavelengths use a reflective, rather than a transmissive, pattern on the mask because the substrate is not sufficiently transparent to such small wavelengths of radiation. In such masks, radiation is reflected from the mask onto the photoresist.
The photolithography mask must be free of manufacturing imperfections if the mask is to accurately produce the desired exposure pattern. Most newly fabricated masks have defects such as missing or excess pattern material and, before such masks can be used, the defects are repaired, often by using a charged particle beam system to remove or deposit material onto the mask substrate.
The requirement for smaller wafer features in photolithography places ever-increasing demands upon the three-dimensional structuring capabilities of the techniques used to repair defects on the photomasks. Repair strategies for clear defects on chrome binary-intensity-masks (BIM) and molybdenum-silicide attenuated-phase-shift-masks (MoSi PSM) are typically based upon reconstructing as closely as possible the original physical structure of the mask feature along with the optical properties of the materials. While direct replacement, i.e., the substitution of a void with the original mask material, is the most straightforward approach to clear defect repair, a number of practical considerations greatly limit the optical fidelity of this repair strategy. For example, the patching of clear defects, that is, missing absorber or phase shifting material, on both BIM and PSM by focused ion beam (FIB) induced deposition typically does not employ the native masking material but rather a carbon-based material whose height is adjusted to mimic the desired optical properties. In the case of MoSi PSM, the deposited height can satisfy only one of the designed values for phase and transmission; in practice the latter is matched due to the ease of measurement although the former is more important for sharpening the edge transition. Furthermore, the fabrication of a structure by FIB-induced deposition of material from a gas phase onto the surface is a complicated process, which is very difficult to control on the nanometer scale.
In the case of an opaque defect, that is, the presence of extra absorber or phase shift material, the defect can be repaired by removing the extra material using charged particle beam, for example, a focused beam of gallium ions. Unfortunately, the ion beam also damages the mask surface and implants ions into the substrate, which adversely affects the transmission of light through the substrate. As shorter wavelengths are used in photolithography, imperfections in the substrate have a greater effect on the aerial image of the mask. Any alteration of the substrate caused by the repair affects the mask performance, so new mask repair systems are needed that will reduce the effect on the substrate.
One method of reducing the effects of ion beam mask repair entails scanning a charged particle beam across the repaired area in the presence of an etchant gas, such as xenon-difluoride. Such a process is described, for example, in U.S. Pat. No. 6,042,738 to Casey, Jr. et al. The clean-up step described in U.S. Pat. No. 6,042,738 adds an extra step to the mask repair process, and the results may still not be comparable to an area that was originally produced without defect. Thus, a method is needed to correct a defective mask so that it projects the desired image onto a work piece.
Therefore, even after a defect is repaired, the repaired area may still have characteristics that are different those of an area that was originally defect-free. For example,